This invention relates to a steam turbine, and more particularly, to a leakage prevention structure for working fluid which is arranged on moving blade tip.
FIG. 9 shows a general steam turbine. A steam turbine 100 has a rotor 2 which is rotatably arranged in a casing 1. The rotor 2 is made to rotate by steam which is working fluid. In the casing 1, nozzle diaphragms 3 are fixed to form static parts together with the casing 1. Each of the nozzle diaphragms 3 has a plurality of nozzle blades 3c which are arranged in the steam path formed between a nozzle diaphragm outer ring 3a and a nozzle diaphragm inner ring 3b, which are annular members, and are arranged in the circumferential direction. The nozzle diaphragm outer ring 3a is fixed to the casing 1, and is substantially concentrically arranged with respect to the rotor 2.
On the outer circumference part of the rotor 2, at positions adjacent to the nozzle diaphragms 3 in the axial direction, a plurality of moving blades 4 are arranged in the circumferential direction with intervals provided therebetween, and configure a rotation part together with the rotor 2. Each of the moving blades 4 has an implantation part 4a, a moving blade effective part 4b, and a moving blade tip 4c. The implantation parts 4a are engaged with the outer circumference part of the rotor 2 to be implanted thereto. The moving blade effective parts 4b are arranged in the steam path. Steam outflowing from the nozzle blades 3c passes through the space around the moving blade effective parts 4b to perform work and generate rotational force. The moving blade tips 4c are structural members which are arranged on the outer circumference part of the respective moving blades 4. The moving blade tips 4c are in contact with the moving blade tips 4c of the adjacent moving blades 4 in the circumferential direction to form an annular member as a whole, and play a role of fixing the tips of the moving blade effective parts 4b. The nozzle diaphragm outer ring 3a is arranged to be extended to the moving blade tips 4c of the moving blades 4, and faces the moving blade tips 4c in the radial direction.
In the steam turbine 100, the paired nozzle diaphragm 3 and moving blades 4 form a turbine stage. Steam supplied to the steam turbine 100 is directed to the space between the nozzle blades 3c of the nozzle diaphragm 3 and has its flowing direction changed, and then is directed to the space between the moving blade effective parts 4b of the moving blades 4 to generate rotational force to the moving blades 4 and the rotor 2. In the steam turbine 100 shown in FIG. 9, there are shown two turbine stages each formed by a nozzle diaphragm 3 and moving blades 4, and the nozzle diaphragms 3 of the two stages are coupled by bolts 9 to be arranged.
In the steam turbine 100, between the rotation part formed by the rotor 2 and moving blades 4, and the static part formed by the casing 1 and nozzle diaphragms 3, flow of leakage is generated. When the amount of the flow leakage is high, the efficiency and output of the steam turbine 100 is lowered. Accordingly, it is required to reduce the clearance provided between the rotation part and the static part as much as possible. For this reason, there is a known structure in which, on the outer circumference part of the moving blade tips 4c of the moving blades 4, seal strips 4d which protrude in the radial outward direction and are arranged in the form of a circumference are provided, which reduces the clearance provided between the tip of the seal strips 4d and the nozzle diaphragm outer ring 3a facing the seal strips 4d as much as possible, suppressing the flow leakage. Furthermore, there is also known a structure in which, on the surface of the nozzle diaphragm outer rings 3a facing the seal strips 4d, a coating layer (abradable layer 3d) made of an abradable material being a free-machining material etc. is arranged, which makes the seal strips 4d cut the abradable layer 3d, making it possible to further reduce the clearance to suppress the amount of the flow leakage.
In the steam turbine, since the rotor and casing are heated to be deformed in the transient operation at the time of the start up and shut down, it is impossible to set up the clearance between the rotation part and the static part to the minimum by only taking the rated operation time into consideration. Furthermore, in case a contact is raised between the rotation part and the static part during the operation, the seal strips may be damaged due to the contact. In some cases, the seal strips may be seriously damaged. Therefore, it is desired to set up a configuration in which the seal structure can be repaired.
As a seal structure that reduces a flow leakage by employing the seal strips and abradable layer, there is conventionally known a technique which is disclosed in Japanese Patent Application Publication No. 2003-65076 (the entire content of which is incorporated herein by reference). In this conventional technique, on the inner circumference side of the nozzle diaphragm outer ring which faces the seal strips arranged on the moving blade tips, a plurality of seal support member segments, each in the form of an arch, having the abradable layer are attached via springs. Employing this configuration, during the transient state of the turbine at the times of starts and stops, it becomes possible to shift the seal support member segments having the abradable layer in the radial outward direction.
However, under the seal structure using the seal strips and abradable layer of the conventional technique, the seal support member segments having the abradable layer are engaged with the nozzle diaphragm via springs, and are so arranged as to be able to shift in the radial direction. Accordingly, when seal fins come into contact with the abradable layer, especially in the transient state of the turbine at the times of starts and stops, there is raised an unstable behavior in which the seal support member segments jounce in the radial direction, which may raise a possibility that the seal fins and the abradable layer come into contact with each other widely and sometimes deeply. In this way, when the seal strips and the abradable layer come into contact with each other, there is a problem that, in the steady operation, the clearance at this part becomes large to increase the leak steam amount, and, furthermore, depending on the way of contact, the seal strips and abradable layer may be damaged.
Furthermore, under the seal structure of the conventional technique, since the seal support member segments are engaged with the nozzle diaphragm outer ring via springs such that the seal support member segments can shift in the radial direction, there is a disadvantage that, so as to keep the structural intensity of the nozzle diaphragm outer ring sufficiently, the nozzle diaphragm outer ring becomes large.
To prevent this problem, without employing the configuration in which the seal support member segments are engaged with the nozzle diaphragm outer ring via springs, as shown in FIG. 9, it can be considered that the abradable layer is directly arranged on the surface of the nozzle diaphragm outer ring 3a facing the seal strips 4d by the coating etc. By employing this configuration, the abradable layer 3d does not shift in the radial direction, which can reduce the part to be scraped away by the seal strips 4d to the minimum, making it possible to reduce the flow leakage. However, in the configuration shown in FIG. 9, since the nozzle diaphragm outer ring 3a of the respective stages having the abradable layer arranged on the inner circumference surface thereof is coupled by the bolts 9 to be unitedly formed, in case the seal strips 4d of a stage come into contact with the abradable layer 3d to damage the abradable layer 3d, the abradable layer 3d has to be repaired after detaching the entire nozzle diaphragm 3 of the stage, which raises another problem of making it difficult to repair the seal structure.